As others have recommended: Just keep a dry set of clothes at work (or tote them with you). Whether it will rain or not is more predictable than the localized intensity of the rain as you pass through it. I can ride a half hour in light rain and not get very wet, but 30 seconds in an unexpected downpour has me drenched. I know what you mean about rain pants. 50 deg F is probably still too warm to prevent condensation.

Good luck, and remember Doppler radar is your friend.